The primary role of any progressive demagnetization technique is to isolate the different components of magnetization recorded by a paleomagnetic sample. Thermal demagnetization achieves this by heating the sample to progressively higher temperatures. Heating a sample in zero magnetic field unblocks magnetic grains that have unblocking temperatures below the heating temperature. The physical mechanism of thermal demagnetization is the same as TRM acquisition. Progressive demagnetization data are typically plotted using orthogonal vector component diagrams[1] and equal area sterograms. The data can then be analyzed using, for example, Principal Component Analysis of linear best-fits to the respective magnetization components of interest[2] .
Potential problems
A commonly encountered problem with thermal demagnetization is thermally-induced alteration: as an effect of heating, magnetic minerals may form due chemical reactions within the sample, or between the sample and the atmosphere. Magnetite is a common product of these reactions, and can produce spurious remanences which obscure the original magnetization of the sample. Mineral alteration can often be detected by measuring the magnetic susceptibility of a sample between each heating step; a sharp increase in susceptibility indicates that a new magnetic mineral has formed.
References
^ Zijderveld, J. D. A. (1967), A.C. demagnetization of rocks: Analysis of results, in Methods in Palaeomagnetism, edited by D. W. Collinson, K. M. Creer, and S. K. Runcorn, pp. 256–286, Elsevier, New York.
^ Kirschvink, J. L. (1980), The least-squares line and plane and the analysis of palaeomagnetic data, Geophys. J. R. Astr. Soc., 62, 699–718, doi: 10.1111/j.1365-246X.1980.tb02601.x.
Thermal Demagnetization
Table of Contents
Potential problems
A commonly encountered problem with thermal demagnetization is thermally-induced alteration: as an effect of heating, magnetic minerals may form due chemical reactions within the sample, or between the sample and the atmosphere. Magnetite is a common product of these reactions, and can produce spurious remanences which obscure the original magnetization of the sample. Mineral alteration can often be detected by measuring the magnetic susceptibility of a sample between each heating step; a sharp increase in susceptibility indicates that a new magnetic mineral has formed.References
Further Reading
Can be removed if not needed.
See Also
Alternating Field (AF) Demagnetization
Chemical Demagnetization
Orthogonal vector Component Diagrams
Principle Component Analysis (PCA)
Thermoremanent Magnetization (TRM)